An important class of hematopoietic interactions occurs between proliferating hematopoietic cells and other parenchymal or stromal cells of the blood forming tissues. This class of interactions is also the least understood. The objectives of this study are to determine the morphological characteristics, evaluate the roles, and define the interactions of stromal cells of the hematopoietic microenvironment which influence or regulate hematopoietic stem cells and progenitor cells. Monolayers of stromal elements from various hematopoietic organs will be established as primary cultures. Enriched and/or cloned populations of stromal cells will then be derived from these heterogenous primary monolayers. Cells obtained in primary, enriched and cloned cultures will then be studied utilizing both in vivo and in vitro techniques. Monolayers will be tested for their ability to support CFU-S and CFU-C in the Dexter liquid culture system. Conditioned media will be analyzed routinely for the presence of humoral regulators of hematopoiesis (CSF and BPA) using both bioassay and radioimmunoassay techniques. Cells harvested from monolayers will be transplanted back to an in vivo site in normal or hematopoietically stressed hosts to assess their ability to participate in the reformation of a functional hematopoietic microenvironment. Mixtures of cloned or enriched stromal cells will also be employed in both the in vivo and in vitro systems to address the problem of multiple cell interactions in hematopoiesis. Stromal cell grafts as well as monolayers will be evaluated using routine histochemical and histophysiological techniques at both the LM and EM level. Chromosome markers will be used to investigate the origin of both the stromal cell and hematopoietic cell populations found in the graft. Finally, stromal cell populations will be cultured in matrix systems to study their ability to form a functional three-dimensional hematopoietic microenvironment in vitro. This study will provide information that is relevant not only to our understanding of mammalian cell biology and in particular cellular hematology, but also potentially useful in clinical hematology.